Regeneration of spent doctor



Feb. 7, 1939. c. L. READ ET AL REGENERATION OF S PENT DOCTOR I Filed on;3; 1935 RE GENERATED .DOOTOR OUTLET .3577'1. l/VG TANK 4 G11 ll .II F 2j 4/ M 5 m A E M M pee-r02 .SOLUT/ON sulfide on addition of sulfur.

Patented Feb. 7, 1939 UNITED STATES PATENT OFFICE REGENERATION OF SPENTDOCTOR Application October 3, 1935, Serial No. 43325 4 Claims.

This invention relates to improvements in the regeneration of spentdoctorsolution, especially spent doctor solution that had been used insweeteninggasoline or naphtha, etc.

Most of the gasoline and naphtha marketed is finished .by doctorsweetening. Doctor sweetening consists essentially in converting themercaptans to lead mercaptides by treating. with a sodium plumbitesolution followed by the conversion of the mercaptides to disulfides andlead Some of the doctor solution readily separates from the oil as aclear solution while another fraction of the doctorsolution separatesfrom the major portion of the oil in .the form of an emulsion consistingof about 25 to 85% of naphtha, 20 to 50% of spent doctor solution, and 5to 20% of lead sulfide.

It was the practice formerly when it was desired to regenerate thedoctor solution toblow the entire spent doctor solution with air,whereby the more volatile naphtha portions were completely lost afterprolonged blowing with air, the lead sulfide converted to lead oxide andregen erated sodium plumbite solution recovered. A

"' more recent practice is to first separate out the naphtha by means ofa centrifuge and blow the residue consisting of leadsulfide and spentdoctor solution with air .to regenerate sodium plumbite solution. Adisadvantage in this process is in the length of time requiredtoregenerate the spent doctor solution.

An object of this invention is to provide an improved method ofregenerating spent doctor solution in a short period of time.

This and other objects of the invention will be understood on readingthe following description with reference to the accompanying drawmg:

Fig. I is a somewhat diagrammatic representa- ..-tion in elevation of anapparatus adapted for the practice of the improved process of theinvention;

Fig. II is an elevation partly in section of one of the forms ofapparatus suitable for use in the apparatus illustrated in Fig. I;

Fig. III is a view of the rotor in the apparatus illustrated in Fig. II;

Fig. IV is a view of the stator in the apparatus illustrated in Fig. II;and

Fig. V is a downward view of the apparatus illusstrated in Fig. II,without the motor, rotor and stator.

Referring to the drawing, numeral l denotes a pipe provided with valve 2through which the spent doctor solution together with lead sulfide(after the naphtha has been removed) is passed into the lower part of amechanical mixer 3 into a rotor 4. In the mechanicalmixer 3, the rotor 4is revolved by means of electric motor 5. This rotor is revolved in thespace enclosed by the stator 6, both the rotor and stator being in turnenclosed by a cylinder 1! which is enclosed on the bottom with a plate8, and may also be enclosed on the top with a plate 9.

The spent doctor solution together with the lead sulfide, by means ofthe revolving rotor 4 is brought in intimate contact with air or otheroxygen containing gas passing through openings (not shown) in the plate9 into the space enclosed by thestator 6. The rapid movement of thespent doctor solution by means of the rotor l creates a partial vacuumwhich causes a large volume of air to rush into the rotor. Thein-rushing air is there thoroughly dispersed through the spent doctorsolution in the form of fine bubbles and regenerates the doctorsolution. When the spent doctor solution is subjected to this mechanicalstirring, the fine bubbles of air are dispersed through this solution,and at times a considerable quantity of foam may be produced. The foammay be several feet in depth and is removed by means of spillway ll,together with regenerated doctor solution, through an opening in theside of cylinder wall Spillway ll empties the regenerated doctorsolution and the foam into settling tank l2, which is provided with acone-shaped bottom I3. The foam readily breaks and two layers areformed, one of regenerated doctor solution and the other of lead sulfidethat has been carried over with the regenerated doctor solution and thefoam. This cone-shaped bottom i3 is provided with an outlet pipe 14,through which, by means of pump I15, the lead sulfide, after settlingout, is returned to the mechanical mixer 3.

As the spent doctor solution may not be all regenerated on passingthrough the mechanical mixer 3 the first time, the spent doctor solutionin the mechanical mixer 3, which has not been withdraw or converted intofoam, flows through openings l6 into a space I! enclosed by bottom plate8 and bottom enclosure I8. There this spent doctor solution is againrepassed through the rotor 4 and mixed with air.

Spent doctor solution also varies as to the quantities of oxygenrequired for regeneration. Therefore, the process may be varied asillustrated by the following examples:

Example 1 Where only a short time of contact of oxygen 'removed to thesettling tank.

and spent doctor is required for regeneration of the doctor solution,the spent doctor solution is only run through the mechanical mixer once.The liquid and any foam that may have been formed is drained offcontinuously to the settling tank. The doctor solution in the settlingtank being regenerated is drained off and the lead sulfide that settlesout is returned to the system, and on mixing with a fresh quantity ofspent doctor solution it is regenerated.

Example 2 Where only a short time of contact of oxygen 3 in spent doctoris required but excessive foaming takes place, the spent doctor solutionis circulated through the mechanical mixer and foam continuouslywithdrawn. The foam on settling breaks 7 to form a liquid layer ofregenerated doctor solution and a solid layer of powdered lead. The leadsulfide is separated from the regenerated doctor solution and returnedto the system for further treatment. 7

Example Where a longer period of contacting oxygen and spent doctorsolution is required for regeneration and excessive foaming takes place,the spent doctor solution is circulated through the mechanical mixer andthe foam that is formed The foam in settling breaks down toa liquidlayer and a solid layerof powdered lead sulfide, which are both returnedto the mechanical mixer until a regenerated doctor solution is obtained.

It has been found in using a regenerating unit of 6 gallons capacitywith a settling tank, that it is possible to regenerate from to 30gallons per hour of the spent doctor solution and at the same timeincrease the lead content of the doctor solution from about .01 poundper gallon to approximately .12 pound per gallon in one hour. 7

In this process it has beenfound that the lead is more rapidly oxidizeddue to the rapid circulation which keeps the lead sulfide in suspensionand' thereby aids oxidation. On account of the rapid circulation, theliquid can carry large percentages of solids, 10 to in suspension andthereby greatly increases the conversion of lead sulfide to lead oxide.

The foregoing description is merely illustrative, and various changesmay be made with the scope of the appended claims, in which it is ourintention to claim all novelty as broadly as the prior art permits.

We claim:

1. A process for regenerating spent doctor solution which comprisesintroducing said solution, togetherwith lead sulfide, into a,regenerating zone, introducing a large volume of oxygen-containing gasinto said solution in said zone while vigorously agitating the mixturewhereby a large quantity of foam is produced, withdrawing all of theregenerated doctor solution and the foam from the upper part of the saidregenerating zone exclusively and conducting it to a zone of largercapacity in which the foam breaks allowing lead sulfide containedtherein to settle to the bottom of said second zone and returning thelead sulfide to the regenerating zone.

2. A process for'regenerating spent doctor solu- V tion which comprisesintroducing it, together with lead sulfide, into a regenerating zone,introducing oxygen,containing gas into the mixture Whilethoroughlyagitating the same whereby a large quantity of foam isproduced, removing all of the foam and regenerated doctor solution asthey are formed from the upper part only of said regenerating zone intoa settlingzo ne in which the foam breaks and allows lead sulfide tosettle, returning the lead sulfide to the regenerating zone and removingthe regenerated doctor solution from said settling zone. l

3. A process for regenerating spent doctor solution which comprisescontinually passing a mixture of said solution, together with leadsulfide, through a regenerating zone, introducing oxygencontaining gasinto said mixture and mechanically agitating the same vigorously whileit is in said zone whereby a large quantity of foam is produced, passingsaid mixture, together with all the foam produced from the upper part ofsaid regenerating zone exclusively, into a separating zone in which thefoam breaks, and,

air and said mixture and producing large quantitles of foam, conductingall of the foam and doctor solution from the upper part of theregenerating zone exclusively, into a separate settling zone where thefoam is permitted to break and the suspended lead sulfide settles, andrecycling the lead sulfide to the regenerating zone.

CHESTER L. READ. THOMAS J. BROIDRICK.

